1. Field of the Invention
The present invention relates to analogs of 2′,5′-oligoadenylate (2-5A) that are stable and have superior activity (particularly antitumor activity).
2. Background Art
2-5A, which is known as a biological substance that has antiviral activity (Pharmacol. Ther. Vol. 78, No. 2, pp. 55-113, 1998), is a short-chain oligonucleotide composed of three or more adenosine units in which two adenosine 2′ and 5′ hydroxyl groups are linked with phosphate 2′,5′-phosphodiester bonds, and in which a triphosphate group is bonded to the 5′ end. When cells infected by a virus are subjected to extracellular interferon stimulation, 2-5A synthetase is induced in the presence of viral dsRNA, and 2-5A is produced from ATP. 2-5A is a substance that converts the inactive form of the RNA degrading enzyme, RNase L, into the active form within host cells. This activated RNase L inhibits viral growth in cells by degrading viral RNA. Moreover, when ovarian cancer cells Hey1B are transfected with 2-5A, sequence-specific cleavage of 18S rRNA is known to occur, that results in demonstration of antitumor activity as a result of apoptosis through release of cytochrome c and activation of caspase (J. Interferon Cytokine Res., 20, 1091-1100 (2000)). Thus, 2-5A is expected to act as a virus growth inhibitor, and, more specifically, as an antivirus drug or antitumor drug.
In an in vitro experiment, an oligonucleotide composed of three or more adenosine units having a monophosphate group on the 5′ end and linked with 2′-5′ phosphodiester bonds is known to activate RNase L (Pharmacol. Ther. Vol. 78, No. 2, pp. 55-113, 1998; J. Biol. Chem. Vol. 270, No. 11, pp. 5963-5978 (1995)). However, 2-5A itself is easily degraded to AMP and ATP by 2′-phosphodiesterase and nuclease. Moreover, the 5′-phosphate group or 5′-triphosphate group ends up being dephosphorylated by phosphatases in the living body and losing activity. Thus, in the case of using 2-5A as a virus growth inhibitor or antitumor drug, a 2-5A analog is desirable that has similar activity, but has high stability, making it more resistant to degradation and metabolism in the living body.
In order to overcome these shortcomings, various methods have been attempted as examples of modifying the phosphate groups. Examples of known methods include a method in which the non-bridging oxygen atom bonded to the phosphorus atom of the phosphodiester bond of the oligonucleotide is substituted with a sulfur atom (phosphorothioate modification), a method in which said oxygen atom is substituted with a methyl group, a method in which said oxygen atom is substituted with a boron atom, and a method in which the sugar portion or nucleobase portion of the oligonucleotide is chemically modified (Freier, S. M.; Altmann, K. H., Nucleic Acids Res., 25, 4429 (1997)). A known example of such a 2-5A analog is the adenosine tetramer which has undergone the phosphorothioate modification shown below (Carpten, J. et al. Nature Genetics, 30, 181 (2002)).

Moreover, analogs having a chemical structure like that shown below, in which the sugar portion of adenosine has been modified, are described in Japanese Patent Application (Kokai) No. Hei 10-195098 and Japanese Patent No. 3420984 as adenosine units of 2-5A analogs.

In the above formula, Y1 and Y2 represent a hydrogen atom or a protecting group for a hydroxy group, and A represents an alkylene group having from 1 to 3 carbon atoms.
In addition, a 2-5A molecule bonded by means of a linker with an antisense molecule in the form of an oligonucleotide having a sequence complementary to mRNA involved in diseases has been used as a 2-5A antisense oligonucleotide that inhibits the function of mRNA (S. A. Adah, et al., Current Medicinal Chemistry (2001), 8, 1189-1212). A highly stable 2-5A analog that is resistant to degradation and metabolism in the living body serves as a portion of a superior 2-5A antisense oligonucleotide, and is expected to be a useful drug. In particular, oligonucleotides containing a bridged nucleoside in which an oxygen atom at the 2′ position and a carbon atom at the 4′ position of the sugar portion are bonded with an alkylene group are known to be useful as antisense molecules (Japanese Patent Application (Kokai) No. Hei 10-304889, Japanese Patent Application (Kokai) No. 2000-297097).